An FM radio receiver, in general, comprises a front end including a tuner, intermediate frequency amplifier/detector, stereo demodulating stage and audio stage. Japanese Patent Publication No. 15499/82 proposes a circuit which is used in such an FM radio receiver to automatically control the stereo demodulating stage and reduce a noise entered therein.
FIG. 4 is a block diagram of an FM radio receiver of this type in which reference symbol ANT designates an antenna, 1 refers to a front end, 2 to an intermediate frequency (IF) amplifier/detector, 3 to a stereo demodulating stage, 4 to an audio stage, 5 to right (R) and left (L) speakers, and 6 to a control circuit.
Assume now that an FM wave having a frequency f.sub.0 is entered in the receiver of FIG. 4 through the antenna ANT. The entered wave is converted to an intermediate frequency signal having a frequency component of 10.7 MHz by the front end 1. The intermediate frequency component is amplified and FM-detected, and converted to an audio output. If it is a stereophonic broadcasting, the audio output is divided into right (R) and left (L) components by the stereo demodulating stage 3. The audio output is amplified by the audio stage 4 and transmitted to the speaker 5. The IF amplifier/detector 2 also includes a means for generating a d.c. voltage responsive to the field intensity of the FM wave entered in the antenna ANT. The voltage is normally called "signal meter voltage" (hereinafter called "S-meter voltage") because it drives a signal meter provided in the front panel of an FM radio receiver to indicate the field intensity. The stereo demodulating stage 3 effects among other things separation control and high-cut control to reduce multipath noises. The separation control and high-cut control operations are controlled by the S-meter voltage which is closely related to the field intensity.
Assume here that a multipath disturbance occurs in which the FM transmitter signals arrive at the antenna over two or more paths, one directly arriving with a frequency f.sub.0 and the others involving reflections from buildings or other obstacles having the same frequency and arriving later. The S-meter voltage V.sub.S momentarily decreases as shown in FIG. 5, and a ripple component V.sub.P corresponding to the reflections are generated below the reference voltage V.sub.0.
The separation control and high-cut control operations of the stereo demodulating stage 3 serve to improve the signal-to-noise (S/N) ratio upon a rapid degradation thereof due to a decreased field intensity of the FM stereo transmitter signals. More specifically, since the S/N ratio is improved by 22 dB during monophonic reception as compared to stereophonic reception, a great degradation in the S/N ratio can be prevented by separation control and high-cut control responsive to the electric field intensity of the FM stereo transmitter signals. These controls are effected by the control circuit 6. However, the ripple component V.sub.P produced in the negative range during multipath reception as shown in FIG. 5 simply continues for a very limited time, and the voltage V.sub.S immediately returns to the original value V.sub.0. Therefore, it is difficult to effectively activate the separation control and high-cut control in response to the ripple component V.sub.P. In this connection, it is necessary to provide a delay in the change of the voltage V.sub.S following an increase or decrease of the ripple component V.sub.P. More specifically, if a relatively long time is provided between a voltage drop of V.sub.S responsive to a multipath reception and restoration of the original voltage V.sub.0, the separation control and high-cut control can sufficiently follow the change of the ripple component V.sub.P. FIG. 4 shows among others the control circuit 6, which decreases the S-meter control voltage in accordance with the degree of the multipath disturbance.
One arrangement of the control circuit 6 is shown in FIG. 6 in which reference numeral 7 denotes an S-meter voltage input terminal, 8 refers to an amplifier, 9 to a negative rectifier, 10 to an adder, 11 to a separation/high-cut control voltage output terminal, and VR to a variable resistor. The ripple component V.sub.P generated by a multipath disturbance and superposed on the S-meter voltage V.sub.S is extracted via a capacitor C.sub.i and amplified by the amplifier 8. The ripple component V.sub.P from the amplifier 8 is rectified by the rectifier 9 into a negative voltage which in turn is added to the S-meter voltage by the adder 10 to decrease the level of the original S-meter voltage into a control voltage V.sub.C at the terminal 11. The control voltage V.sub.C activates separation and high-cut control operations of the stereo demodulating stage to reduce noises caused by the multipath disturbance and improve the S/N ratio.
FIG. 7 is a circuit diagram of the negative rectifier 9 and adder 10 included in the circuit of FIG. 6. Reference numeral 12 denotes an output terminal of the variable resistor VR (FIG. 6) and 13 designates an output terminal of the amplifier 8. The ripple component caused by the multipath disturbance and amplified by the amplifier 8 passes through a capacitor C.sub.3 and negative-rectified by diodes D.sub.1 and D.sub.2 so as to negatively charge a capacitor C.sub.1. When the difference between the negative voltage of the capacitor C.sub.1 and a voltage at a point B becomes larger than a threshold voltage V.sub.D of a diode D.sub.3, the capacitor C.sub.2 discharges through a resistor R.sub.1 and the diode D.sub.3. As the result, a current flows through resistors R.sub.3 and R.sub.2 to charge the capacitor C.sub.2, and decreases the voltage V.sub.C at a point C as shown in FIG. 8. Before the multipath disturbance component V.sub.P reaches a degree (a), the diode D.sub.3 maintains the control voltage V.sub.C at V.sub.D, and after the component V.sub.P exceeds (a), the control voltage decreases due to conduction of the diode D.sub.3. When the multipath disturbance component V.sub.P is (b), the control voltage V.sub.C is V.sub.1. In this case, the control voltage V.sub.C may take different values V.sub.2, for example, in addition to V.sub.1 at a fixed degree of the multipath disturbance component V.sub.P, depending on the gain of the amplifier 8. That is, a voltage change ratio .gamma..sub.0, i.e.: ##EQU1## varies depending on the gain of the amplifier as represented by: ##EQU2## The gain of the amplifier 8 cannot be changed so much due to a restriction of a d.c. amplification ratio h.sub.FE. Also, the use of a larger resistance in the adder 10 to increase the voltage drop invites an influence to the time constant fixed for a delayed charging of the capacitor C.sub.2. Therefore, free selection of the voltage change ratio .gamma..sub.0 cannot be expected by changes of the gain of the amplifier 8 nor the resistance R.sub.1 in the adder 10 in the prior art circuit.